Method for Manufacturing Hot-Pressed Member

ABSTRACT

A method for manufacturing a hot-pressed member The method includes heating a zinc-based coated steel sheet to a temperature range from an Ac3 transformation temperature to  1000 ° C., performing hot pressing work, and performing cooling, in which a surface-cleaning treatment is performed on the zinc-based coated steel sheet before the heating step is performed.

TECHNICAL FIELD

The present disclosure relates to a method for manufacturing ahot-pressed member which can preferably be used for, for example,chassis and body structure members of an automobile.

BACKGROUND ART

To date, for example, chassis and body structure members of anautomobile have usually been manufactured by performing press working onsteel sheets having a specified strength. Nowadays, since there is astrong demand for the weight reduction of an automobile body from theviewpoint of global environment conservation, efforts are being made toreduce the thickness of a steel sheet which is used for an automobilebody by increasing the strength of the steel sheet. However, an increasein the strength of steel sheets causes a deterioration in pressworkability, and therefore there is an increase in the number of caseswhere it is difficult to form a steel sheet into desired shapes of themembers.

In view of the situation described above, Patent Literature 1 proposes aworking technique called hot pressing which makes it possible to realizean improvement in workability and an increase in strength at the sametime by performing working and rapid cooling at the same time on aheated steel sheet by using a mold which is composed of a die and apunch. However, the case of this hot pressing, since a steel sheet isheated to a high temperature of about 950° C. before hot pressing isperformed, scale (iron oxide) is generated on the surface of the steelsheet, and the flaking of the scale occurs when hot pressing isperformed, which results in a problem in that a mold is damaged or thesurface of a member is damaged after the hot pressing. Also, scale whichremained on the surface of a member causes a deterioration in surfaceappearance and paint adhesiveness. Therefore, scale which is present onthe surface of a member is usually removed by performing a treatmentsuch as pickling or shot blasting. However, such a treatment makes amanufacturing process complex, and thus there is a decrease inproductivity.

Moreover, for example, chassis and body structure members of anautomobile are also required to have excellent corrosion resistance.However, since a hot-pressed member, which is manufactured by using theprocess described above, is not provided with an anti-corrosion filmsuch as a coating layer, the member is very poor in terms of corrosionresistance.

Therefore, a hot-pressing technique is desired with which the generationof scale can be inhibited when heating is performed before hot pressingand with which the corrosion resistance of a hot-pressed member can beimproved, and thus a steel sheet to be hot-pressed whose surface iscoated with a film such as a coating layer and a method for hot pressingwhich utilizes the steel sheet have been proposed.

For example, Patent Literature 2 discloses a method for manufacturing ahot-pressed member excellent in terms of corrosion resistance whosesurface is coated with a Zn—Fe-based compound or a Zn—Fe—Al-basedcompound by performing hot pressing on a steel sheet, which is coatedwith Zn or a Zn-based alloy.

In addition, Patent Literature 3 discloses a hot-pressed memberexcellent in terms of scale resistance, paint adhesiveness,after-painting corrosion resistance, and hydrogen entry resistance whichis manufactured by forming a Ni diffusion region, a layer of anintermetallic compound, which is equivalent to the γ phase of a Zn—Nialloy, and a ZnO layer in the surface layer of a steel sheet.

Moreover, Patent Literature 4 discloses a steel sheet to be hot-pressedwhich is manufactured by forming a ZnO layer, which inhibits thevaporization of Zn, in the surface layer of a Zn-based coating layer inadvance, and indicates that it is possible to obtain a hot-pressedproduct having good surface appearance, excellent paint adhesiveness,and excellent after-painting corrosion resistance by using the steelsheet.

CITATION LIST Patent Literature

PTL 1: UK Patent No. GB1490535

PTL 2: Japanese Patent No. 3663145

PTL 3: Japanese Patent No. 4849186

PTL 4: Japanese Patent No. 3582511

SUMMARY Technical Problem

As disclosed in the conventional techniques described above, using azinc-based coated steel sheet as a steel sheet to be hot-pressed iseffective for improving corrosion resistance. However, since the meltingpoint of zinc is 419° C. and the boiling point of zinc is 907° C., thatis, since both are low, the melting of zinc in the coating layer and thevaporization of zinc from the coating layer occur in a heating processbefore hot pressing, which makes it difficult to stably manufacture ahot-pressed member having a homogeneous and good surface appearance.

For example, in the case of a hot-pressed member which is manufacturedby using the method described in Patent Literature 2, since azinc-coated steel sheet or a zinc-aluminum-coated steel sheet, whosecoating layer has a low melting point, is used, the melting of thecoating layer or the vaporization of zinc which occurs in a heatingprocess before hot pressing is significant. As a result, a hot-pressedmember, which is finally obtained, has, for example, a spottyinhomogeneous surface appearance or many white or black point-likedefects. Therefore, it is difficult to obtain a hot-pressed memberhaving a homogeneous and good surface appearance. Here, since not only adeterioration in surface appearance but also a deterioration in paintadhesiveness occur in a portion in which point-like defects exist, thereis a strong demand for a technique for preventing point-like defects.However, an effective technique has not been proposed.

In the case of a hot-pressed member described in Patent Literature 3which is manufactured by using a steel sheet coated with a Zn—Ni-alloycoating layer, which has a melting point higher than that of Zn, thereis an improvement in the surface appearance of a hot-pressed membercompared with the case where a zinc-coated steel sheet or azinc-aluminum-coated steel sheet is used. However, it is not possible tocompletely prevent the generation of local point-like defects.

In the case where a steel sheet to be hot-pressed described in PatentLiterature 4 is used, there is an improvement in the surface appearanceof a hot-pressed member to some extent through the effect of a ZnO layerwhich is formed in the surface layer of the steel sheet. However, thereis still a problem in that local point-like defects are generated in aportion in which the result of a treatment for forming a ZnO layer isinhomogeneous. In addition, there is a problem of a significant increasein cost when a treatment for forming a ZnO layer is performed, becauseit is necessary to use a method which involves, for example, anoxidizing treatment through the use of heat, a contact treatment with asolution, an electrolysis treatment in an aqueous solution, and atreatment of coating and drying a solution.

The present disclosure has been completed in order to solve the problemsof the conventional techniques described above, and an object of thepresent disclosure is to provide a method for manufacturing ahot-pressed member excellent in terms of surface appearance with whichit is possible to stably manufacture a hot-pressed member having ahomogeneous and good surface appearance without causing a significantincrease in cost.

Solution to Problem

The present inventors, in order to solve the problems described above,diligently conducted investigations regarding a method for manufacturinga hot-pressed member excellent in terms of surface appearance. First,investigations regarding a state in which point-like defects aregenerated on the surface of a hot-pressed member were conducted. As aresult, it was found that the position or number of point-like defectsis not necessarily the same even if the same kind of zinc-based coatedsteel sheet is heated under the same condition. Also, it was found thatit is not possible to completely prevent point-like defects even with aZn—Ni-alloy-coated steel sheet, although a steel sheet coated with aZn—Ni-alloy coating layer, which has a higher melting point than that ofzinc-based coated steel sheet, can be used as a steel sheet to behot-pressed more advantageously than a zinc-based coated steel sheetsuch as a galvanized steel sheet or a galvannealed steel sheet, whichhas a coating layer having a comparatively low melting point, from theviewpoint of inhibiting point-like defects from occurring.

From such facts, on the basis of the hypothesis that the generation ofpoint-like defects depends not only on the kind of a coating layer andheating conditions but also on other factors such as a stain on thesurface, verification experiments were conducted in which a zinc-basedcoated steel sheet was heated after the surface of the steel sheet hadbeen stained or cleaned. As a result, it was found that point-likedefects, which are generated on the surface of a hot-pressed member, arecaused by stains due to, for example, dirt, dust, and fingerprint whichare adhered to the surface of a coating layer. That is, it was foundthat there is a local increase in temperature due to extraneous matterderived from such stain materials being burned in a heating processbefore hot pressing work, which results in the vaporization of zincbeing promoted due to the breakage of a ZnO layer with which the surfaceof the zinc-based coating layer has been covered, and which results inscale being generated in such a portion. In addition, it was furtherfound that it is possible to significantly inhibit the generation ofpoint-like defects by performing a surface-cleaning treatment in orderto remove such stain materials before a heating process. Moreover, itwas found that the effect of inhibiting point-like defects through theuse of a surface-cleaning treatment before heating is realized not onlyin a process in which a zinc-based coated steel sheet is subjected tohot pressing work and cooling after having been heated but also in aprocess in which a zinc-based coated steel sheet is heated and cooledafter having been subjected to cold pressing work.

The method for manufacturing a hot-pressed member according to thepresent disclosure has been completed on the basis of such findings.

[1] A method for manufacturing a hot-pressed member having an excellentsurface appearance, the method including heating a zinc-based coatedsteel sheet to a temperature range from the Ac3 transformationtemperature to 1000° C., performing hot pressing work, and performingcooling, in which a surface-cleaning treatment is performed on thezinc-based coated steel sheet before the heating is performed.

[2] A method for manufacturing a hot-pressed member having an excellentsurface appearance, the method including performing cold pressing workon a zinc-based coated steel sheet, heating the zinc-based coated steelsheet to a temperature range from the Ac3 transformation temperature to1000° C., and cooling the heated steel sheet, in which asurface-cleaning treatment is performed on the zinc-based coated steelsheet before the heating is performed.

[3] The method for manufacturing a hot-pressed member according to item[1] or [2], in which the zinc-based coated steel sheet is aZn—Ni-alloy-coated steel sheet having a coating layer on one or bothsides of the Zn—Ni-alloy-coated steel sheet, and the coating layer has achemical composition containing 10 mass % to 25 mass % of Ni and thebalance being Zn and inevitable impurities and a coating weight per sideof 10 g/m² to 90 g/m².

Here, in the present description, % used when describing the chemicalcomposition of steel or a coating layer always means mass %.

Advantageous Effects

According to the present disclosure, it is possible to stablymanufacture a hot-pressed member having a homogeneous and good surfaceappearance without causing a significant increase in cost. Thehot-pressed member which is manufactured by using the present disclosurecan preferably be used for the chassis and body structure members of anautomobile.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating typical examples of the surfaceappearance of a hot-pressed member, where (a) is a photograph indicatinga product having a good surface appearance and (b) is a photographindicating a product having point-like defects.

FIG. 2 is a diagram illustrating typical examples of the surfaceappearance of point-like defects, where (a) is an enlarged photograph ofa white defect and (b) is an enlarged photograph of a black defect.

DESCRIPTION OF EMBODIMENTS

1) Zinc-Based Coated Steel Sheet

In the method for manufacturing a hot-pressed member according to thepresent disclosure, a zinc-based coated steel sheet having a zinc-basedcoating layer on one or both sides thereof is used. Examples of azinc-based coating layer include, but are not limited to, a galvanizinglayer, a galvannealing layer, a hot-dip zinc-aluminum-alloy coatinglayer, a hot-dip zinc-aluminum-magnesium-alloy coating layer, a zincelectroplating layer, and a zinc-nickel-alloy electroplating layer, andall of the known zinc-based coating layers containing zinc may be used.

It is preferable that the coating weight on the surface of such azinc-based coated steel sheet be 10 g/m² to 90 g/m² per side. In thecase where the coating weight per side (hereinafter, also simplyreferred to as “coating weight”) is 10 g/m² or more, the corrosionresistance does not become insufficient. On the other hand, in the casewhere the coating weight is 90 g/m² or less, there is no increase incost. It is more preferable that the coating weight be 20 g/m² to 80g/m². Here, it is possible to determine coating weight by using a wetanalysis method. Specifically, for example, by dissolving the wholecoating layer of a test piece whose coated area is known in an aqueoussolution in which 1 g/L of hexamethylenetetramine is added as aninhibitor to a 6-mass %-hydrochloric acid aqueous solution, the coatingweight of the coating layer should be derived from the amount ofdecrease in mass at this time.

It is preferable that the zinc-based coated steel sheet described abovebe a Zn—Ni-alloy-coated steel sheet having a coating layer on one orboth sides thereof, in which the coating layer having a chemicalcomposition containing 10 mass % to 25 mass % of Ni and the balancebeing Zn and inevitable impurities. In. the case where the Ni content inthe coating layer is 10 mass % to 25 mass %, since the structure of thecoating layer becomes a γ phase structure, and since this γ phase has ahigh melting point of 881° C., there is an increase in the effect ofinhibiting the generation of point-like defects to a higher level. Herea γ phase has one of the crystal structures of Ni₂Zn₁₁, NiZn₃, andNi₅Zn₂₁, and it is possible to identify the structure by using an X-raydiffraction method. Here, an underlying coating layer such as a coatinglayer mainly containing, for example, Ni may be formed under theabove-described coating layer.

2) Base Steel Sheet

In order to obtain the hot-pressed member according to the presentdisclosure, a hot-rolled steel sheet or a cold-rolled steel sheet havinga chemical composition containing, by mass %, C: 0.15% to 0.50%, Si:0.05% to 2.00%, Mn: 0.5% to 3.0%, P: 0.10% or less, S: 0.05% or less,Al: 0.10% or less, N: 0.010% or less, and the balance being Fe andinevitable impurities may be used as a base steel sheet for a zinc-basedcoating layer. In the case where a hot-rolled steel sheet or acold-rolled steel sheet having such a chemical composition is used as abase steel sheet, it is possible to obtain a hot-pressed member having adesired high strength of, for example, 980 MPa or more.

The reasons for the limitations on the constituent chemical elementswill be described hereafter.

C: 0.15% to 0.50%

C is a chemical element which increases the strength of steel, and it isnecessary that the C content be 0.15% or more in order to control thetensile strength (hereinafter, also referred to as “TS”) of ahot-pressed member to be 980 MPa or more. On the other hand, in the casewhere the C content is more than 0.50%, there is a significantdeterioration in the blanking performance of a steel sheet as amaterial. Therefore, it is preferable that the C content be 0.15% to0.50%.

Si: 0.05% to 2.00%

Si is, like C, a chemical element which increases the strength of steel,and it is necessary that the Si content be 0.05% or more in order tocontrol the TS of a hot-pressed member to be 980 MPa or more. On theother hand, in the case where the Si content is more than 2.00%, thereis a significant increase in the amount of a surface defect, which iscalled red scale, generated when hot rolling is performed, there is anincrease in rolling load, and there is a deterioration in the ductilityof a hot-rolled steel sheet. Moreover, in the case where the Si contentis more than 2.00%, there may be a negative effect on coatability when acoating treatment is performed in order to form a coating filmcontaining mainly Zn and Al on the surface of a steel sheet. Therefore,it is preferable that the Si content be 0.05% to 2.00%.

Mn: 0.5% to 3.0%

Mn is a chemical element which is effective for improving hardenabilityby inhibiting ferrite transformation and which is effective fordecreasing a heating temperature before hot pressing is performed,because Mn decreases the Ac₃ transformation temperature. It is necessarythat the Mn content be 0.5% or more in order to realize such effects. Onthe other hand, in the case where the Mn content is more than 3.0%,since Mn is segregated, there is a deterioration in the homogeneity ofthe properties of a steel sheet as a material and a hot-pressed member.Therefore, it is preferable that the Mn content be 0.5% to 3.0%.

P: 0.10% or Less

In the case where the P content is more than 0.10%, since P issegregated, there is a deterioration in the homogeneity of theproperties of a steel sheet as a material and a hot-pressed member, andthere is a significant decrease in toughness. Therefore, it ispreferable that the P content be 0.10% or less.

S: 0.05% or Less

In the case where the S content is more than 0.05%, there is a decreasein the toughness of a hot-pressed member. Therefore, it is preferablethat the S content be 0.05% or less.

Al: 0.10% or Less

In the case where the Al content is more than 0.10%, there is adeterioration in the blanking performance and hardenability of a steelsheet as a material. Therefore, it is preferable that the Al content be0.10% or less.

N: 0.010% or Less

In the case where the N content is more than 0.010%, since nitrides ofAlN are formed when hot rolling is performed or when heating isperformed before hot pressing work is performed, there is adeterioration in the blanking performance and hardenability of a steelsheet as a material Therefore, it is preferable that the N content be0.010% or less.

The remainder is and inevitable impurities. Moreover, it is preferablethat at least one selected from Cr: 0.01% to 1.0%, Ti: 0.01% to 0.20%,and B: 0.0005% to 0.0800% and Sb: 0.003% to 0.030% be added separatelyor at the same time in addition to the chemical composition describedabove for the reasons described below.

Cr: 0.01% to 1.0%

Cr is a chemical element which is effective for increasing the strengthof steel and improving hardenability of steel. It is preferable that theCr content be 0.01% or more in order to realize such effects. On theother hand, in the case where the Cr content is more than 1.0%, there isa significant increase in cost. Therefore, it is preferable that theupper limit of the Cr content be 1.0%.

Ti: 0.01% to 0.20%

Ti is a chemical element which is effective for increasing the strengthof steel and which is effective for increasing toughness by decreasing acrystal grain size. In addition, Ti is a chemical element which iseffective for realizing the effect of improving hardenability throughthe use of solid solution B by forming nitrides more readily than B,which will be described below. Therefore, it is preferable that the Ticontent be 0.01% or more. However, in the case where the Ti content ismore than 0.20%, there is a significant increase in rolling load whenhot rolling is performed, and there is a decrease in the toughness of ahot-pressed member. Therefore, it is preferable that the upper limit ofthe Ti content be 0.20%.

B: 0.0005% to 0.0800%

B is a chemical element which is effective for improving hardenabilitywhen hot pressing is performed and for increasing toughness after thehot pressing. It is preferable that the B content be 0.0005% or more inorder to realize such effects. On the other hand, in the case where theB content is more than 0.0800%, there is a significant increase inrolling load when hot rolling is performed, and, for example, crackingoccurs in a steel sheet due to the formation of a martensite phase and abainite phase after the hot rolling. Therefore, it is preferable thatthe upper limit of the B content be 0.0800%.

Sb: 0.003% to 0.030%

Sb is effective for inhibiting the formation of a decarburized layer inthe surface layer of a steel sheet in a process in which a zinc-basedcoated steel sheet is subjected to heating followed by hot pressing workand cooling. It is necessary that the Sb content be 0.003% or more inorder to realize such an effect. On the other hand, in the case wherethe Sb content is more than 0.030%, since there is an increase inrolling load, there is a decrease in productivity. Therefore, it ispreferable that the Sb content be 0.003% to 0.030%.

3) Hot Pressing Process

In the method for manufacturing a hot-pressed member according to thepresent disclosure, a hot-pressed member is manufactured by using one ofthe processes in the two embodiments described below as a hot pressingprocess. The two embodiments discussed herein are exemplary embodiments.

The first embodiment is a method for manufacturing a hot-pressed memberand is a hot pressing process called direct process in which azinc-based coated steel sheet is subjected to heating to a temperaturerange from the Ac3 transformation temperature to 1000° C. followed byhot pressing work and cooling. In the case where the heating temperatureis lower than the Ac3 transformation temperature, since there is aninsufficient degree of quenching of a steel sheet, there may be a casewhere it is not possible to achieve the desired strength. In addition,in the case where the heating temperature is higher than 1000° C., thereis an economic disadvantage from the viewpoint of energy, and it isdifficult to manufacture a hot-pressed member having a homogeneous andgood surface appearance due to the significant generation of point-likedefects. In addition, cooling after hot pressing work may be performedby using a mold at the same time as hot pressing work, or the coolingmay be performed by using a coolant such as water at the same time ashot pressing work or immediately after the hot pressing work.

The second embodiment is a method for manufacturing a hot-pressed memberand is a hot pressing process called indirect process in which azinc-based coated steel sheet is subjected to cold pressing workfollowed by heating to a temperature range from the Ac3 transformationtemperature to 1000° C. and cooling. In this process, cold pressing workis first performed before a zinc-based coated steel sheet is heated.Subsequently, the cold-pressed member is subjected to heating followedby cooling. The heating temperature is set to be in a temperature rangefrom the Ac3 transformation temperature to 1000° C. for the reasonsdescribed above. Cooling may be performed by using a mold which is usedfor cooling a member or by using a coolant such as water. In addition,when cooling is performed by using a mold, work may be added byperforming hot pressing.

Here, the term “heating temperature” means the maximum end-pointtemperature of a steel sheet. In addition, examples of a method forperforming the heating described above include heating which utilizes,for example, an electric furnace or a gas furnace, direct-fired heating,electrical heating, high-frequency heating, and induction heating.

4) Surface-Cleaning Treatment

In the method for manufacturing a hot-pressed member according to thepresent disclosure, a surface-cleaning treatment is performed on azinc-based coated steel sheet before the zinc-based coated steel sheetis heated in order to remove stains due to, for example, dirt, dust, andfingerprint which are adhered to the surface of a coating layer. Thissurface-cleaning treatment is an important requirement in the presentdisclosure. In the case where this surface-cleaning treatment is notperformed, point-like defects are generated as illustrated in FIG. 1(b).On the other hand, in the case where a surface-cleaning treatment isperformed, it is possible to manufacture a product having a good surfaceappearance as illustrated in FIG. 1(a). Here, examples of point-likedefects include a white defect illustrated in FIG. 2(a), which is a markof intense vaporization of zinc left at the position of the breakage ofa ZnO layer, with which the surface of a zinc-based coating layer hasbeen covered, and a black defect illustrated in FIG. 2(b), which isgenerated as a result of scale being generated through the progress ofthe oxidation of a base steel sheet at the position of a white defect.Since, both of the point-like defects deteriorate surface appearance andpaint adhesiveness, it is necessary to inhibit point-like defects asmuch as possible by performing a surface-cleaning treatment. Asdescribed above, a surface-cleaning treatment according to the presentdisclosure is a treatment which removes the origins of point-likedefects.

It is necessary to perform a surface-cleaning treatment before a heatingprocess. Therefore, in the case of the first embodiment described above(in which a zinc-based coated steel sheet is subjected to heating to atemperature range from the Ac3 transformation temperature to 1000° C.followed by hot pressing work), it is necessary to perform asurface-cleaning treatment on a steel sheet in the form of a coiledsteel sheet or in the form of a cut steel sheet or a steel sheet blank,which has been cut out of the coiled steel sheet. In addition, in thecase of the second embodiment described above (in which a zinc-basedcoated steel sheet is subjected to cold pressing work followed byheating to a temperature range from the Ac3 transformation temperatureto 1000° C.), a surface-cleaning treatment may be performed on a steelsheet in the form of a coiled steel sheet or in the form of a cut steelsheet or a steel sheet blank, which has been cut out of the coiled steelsheet, or on a cold-pressed member after cold pressing work. Here, inany one of the first embodiment and the second embodiment, it ispreferable at the period of time from the end of a surface cleaningtreatment to the beginning of heating be as short as possible. Inaddition, it is preferable that a surface-cleaning treatment beperformed on a steel sheet in the form of a steel sheet blank, becausethis facilitates the treatment with a high level of surface-cleaningeffect.

There is no particular limitation on the method used for performing asurface-cleaning treatment as long as it is possible to remove stainsdue to, for example, dirt, dust, and fingerprint which are adhered tothe surface of a coating layer. Examples of a method for performing asurface-cleaning treatment include one in which the surface of a steelsheet is wiped with waste cloth, one in which the surface of a steelsheet is brushed by using, for example, a nylon brush, one in which thesurface of a steel sheet is brushed after a liquid such as a wash oil,which has no negative effect on a steel sheet, has been applied to thesurface, and one in which alkaline degreasing or solvent degreasing isperformed. Since a method in which a liquid is in contact with thesurface of a steel sheet such as a combined method of wash-oilapplication and brushing, an alkaline degreasing method, or a solventdegreasing method has a higher cleaning effect than a physical methodsuch as one in which wiping with waste cloth or brushing is performed,it is preferable that a method in which a liquid be in contact with thesurface of a steel sheet be performed to clean the surface of a steelsheet completely. However, in the case where alkaline degreasing isperformed as a surface-cleaning treatment by using an alkalinedegreasing liquid having a pH of 12.5 or more, since there is anexcessive effect of dissolving a zinc-based coating layer, thegeneration of point-like defects is conversely promoted due toinhomogeneous dissolution, and there is a deterioration in corrosionresistance due to a decrease in coating weight. Therefore, in the casewhere alkaline degreasing is performed as a surface-cleaning treatmentby using an alkaline degreasing liquid, the pH of the alkalinedegreasing liquid is set to be less than 12.5. Moreover, it ispreferable that the process of a surface-cleaning treatment have a lowcost. In the case where apparatuses for cold pressing work has anapparatus for performing the combination of wash-oil application andbrushing, such equipment can preferably be used for hot pressing work,because it is possible to perform a treatment at low cost with a highcleaning effect by using such equipment.

EXAMPLES

A cold-rolled steel sheet having a chemical composition containing, bymass %, C: 0.23%, Si: 0.25%, Mn: 1.2%, P: 0.01%, S: 0.01%, Al: 0.03%, N:0.005%, Cr: 0.2%, Ti: 0.02%, B: 0.0022%, Sb: 0.008%, and the balancebeing Fe and inevitable impurities, an Ac3 transformation temperature of820° C., and a thickness of 1.6 mm was used as a base steel sheet. Thesurface of this cold-rolled steel sheet was coated with one of agalvanizing layer, a hot-dip Zn—Al-alloy coating layer (Al content: 55mass %), a galvannealing layer (Fe content: 10 mass %), a Znelectroplating layer, and a Zn—Ni-alloy electroplating layer (Nicontent: 12 mass %) and cut into a sample having a size of 200 mm×300mm.

The sample obtained as described above was subjected to asurface-cleaning treatment. A surface-cleaning treatment was performedby using one of A: a wiping method with waste cloth, B: a brushingmethod, C: a combined method of wash-oil application and brushing, D: analkaline degreasing method (pH: 12.0), and E: a solvent degreasingmethod. As comparative examples, samples which had been subjected to F:strong alkaline degreasing (pH: 13.0) and samples which had not beensubjected to a surface-cleaning treatment were also prepared.

Wiping with waste cloth was performed by using waste cloth (CleaningWhite Stockinet Waste (Cotton) produced by Nihon Waste Co., Ltd.) and bywiping the surface of a sample twice with a reciprocating movement of ahand.

Brushing was performed by using a nylon-fiber plant brush (SK-typeStraight Brush produced by Showa Kogyo Co., Ltd.) and by brushing thesurface of a sample twice with a reciprocating movement of a hand.

The combination of wash-oil application and brushing was performed byperforming brushing as described above after having applied a washrust-prevention oil (PRETON R352L produced by Sugimura ChemicalIndustrial Co., Ltd.) to the surface of a sample so that the amount ofoil applied was 2.0 g/m².

Alkaline degreasing was performed by spraying a sample with an alkalinedegreasing liquid (CL-N364S, 20 g/L, 60° C., produced by NihonParkerizing Co., LTD.) for 10 seconds, and thereafter performing waterwashing followed by drying. Here, at this time, the pH of the alkalinedegreasing liquid was 12.0.

Solvent degreasing was performed by immersing a sample in a combinedsolvent of toluene and ethanol having a mixture ratio of 1:1, andthereafter performing ultrasonic cleaning for one minute and drying.

Strong alkaline degreasing, which was performed as a comparativeexample, was performed by immersing a sample in a strong alkalinedegreasing liquid (NaOH aqueous solution, pH: controlled to be 13.0, 50°C.) for seconds, and thereafter performing water washing followed bydrying.

Subsequently, the sample was heated in an electric furnace at atemperature of 950° C. so that duration of the sample in the electricfurnace was 8 minutes, the sample was taken out of the furnaceimmediately after completion of heating, and then held in a flat, moldmade of Al in order to perform a rapid cooling treatment (cooling rate:50° C./s).

The surface appearance of the samples (zinc-based coated steel sheets)obtained as described above was evaluated by using the following method.

Ten samples were each prepared under the same condition in order toincrease the judgment accuracy of surface appearance. After havingperformed a visual test regarding the state of point-like defects of thesamples, surface appearance was judged on the basis of the judgmentcriteria below, and the case of ⊙ or ◯ was judged as satisfactory. Here,in the examples of the present disclosure, although the effect of thepresent disclosure was evaluated on the basis of the evaluation of thesurface appearance after having heated and cooled a flat sheet asdescribed above without performing practical press forming whichutilized a direct process or an indirect process, the results of thepresent evaluation are the same as those of the evaluation of thesurface appearance after having performed practical press forming whichutilizes the above-mentioned two processes, because the surfaceappearance after having performed heating and cooling depends on whetheror not stain components exist on the surface of the sample and on theeffect of removing the stain components.

: 10 out of the 10 samples had no point-like defect

◯: 8 to 9 out of the 10 samples had no point-like defect

Δ: 5 to 7 out of the 10 samples had no point-like defect

×: 0 to 4 out of the 10 samples had no point-like defect

The results of the evaluation of surface appearance are given in Table 1in combination with the manufacturing conditions.

TABLE 1 Coating Layer Coating Steel Weight per Sheet SideSurface-cleaning Treatment Surface No. Type * (g/m²) Code TreatmentMethod Appearance Note 1 GI 60 A Wiping with Waste Cloth ∘ Example 2 GL80 A Wiping with Waste Cloth ∘ Example 3 GA 20 A Wiping with Waste Cloth∘ Example 4 GA 45 A Wiping with Waste Cloth ∘ Example 5 GA 90 A Wipingwith Waste Cloth ∘ Example 6 EG 40 A Wiping with Waste Cloth ∘ Example 7ZN 10 A Wiping with Waste Cloth

Example 8 ZN 60 A Wiping with Waste Cloth

Example 9 ZN 90 A Wiping with Waste Cloth

Example 10 GI 60 B Brushing ∘ Example 11 GL 80 B Brushing ∘ Example 12GA 20 B Brushing ∘ Example 13 GA 45 B Brushing ∘ Example 14 GA 90 BBrushing ∘ Example 15 EG 40 B Brushing ∘ Example 16 ZN 10 B Brushing

Example 17 ZN 60 B Brushing

Example 18 ZN 90 B Brushing

Example 19 GI 60 C Wash-oil Application + Brushing

Example 20 GL 80 C Wash-oil Application + Brushing

Example 21 GA 20 C Wash-oil Application + Brushing

Example 22 GA 45 C Wash-oil Application + Brushing

Example 23 GA 90 C Wash-oil Application + Brushing

Example 24 EG 40 C Wash-oil Application + Brushing

Example 25 ZN 10 C Wash-oil Application + Brushing

Example 26 ZN 60 C Wash-oil Application + Brushing

Example 27 ZN 90 C Wash-oil Application + Brushing

Example 28 GI 60 D Alkaline Degreasing

Example 29 GL 80 D Alkaline Degreasing

Example 30 GA 20 D Alkaline Degreasing

Example 31 GA 45 D Alkaline Degreasing

Example 32 GA 90 D Alkaline Degreasing

Example 33 EG 40 D Alkaline Degreasing

Example 34 ZN 10 D Alkaline Degreasing

Example 35 ZN 60 D Alkaline Degreasing

Example 36 ZN 90 D Alkaline Degreasing

Example 37 GI 60 E Solvent Degreasing

Example 38 GL 80 E Solvent Degreasing

Example 39 GA 20 E Solvent Degreasing

Example 40 GA 45 E Solvent Degreasing

Example 41 GA 90 E Solvent Degreasing

Example 42 EG 40 E Solvent Degreasing

Example 43 ZN 10 E Solvent Degreasing

Example 44 ZN 60 E Solvent Degreasing

Example 45 ZN 90 E Solvent Degreasing

Example 46 GI 60 F Strong Alkaline Degreasing x Comparative Example 47GL 80 F Strong Alkaline Degreasing x Comparative Example 48 GA 20 FStrong Alkaline Degreasing x Comparative Example 49 GA 45 F StrongAlkaline Degreasing x Comparative Example 50 GA 90 F Strong AlkalineDegreasing x Comparative Example 51 EG 40 F Strong Alkaline Degreasing xComparative Example 52 ZN 10 F Strong Alkaline Degreasing Δ ComparativeExample 53 ZN 60 F Strong Alkaline Degreasing Δ Comparative Example 54ZN 90 F Strong Alkaline Degreasing Δ Comparative Example 55 GI 60 — NoTreatment x Comparative Example 56 GL 80 — No Treatment x ComparativeExample 57 GA 20 — No Treatment x Comparative Example 58 GA 45 — NoTreatment x Comparative Example 59 GA 90 — No Treatment x ComparativeExample 60 EG 40 — No Treatment x Comparative Example 61 ZN 10 — NoTreatment Δ Comparative Example 62 ZN 60 — No Treatment Δ ComparativeExample 63 ZN 90 — No Treatment Δ Comparative Example * GI: galvanizinglayer GL: hot-dip Zn-Al-alloy coating layer (Al content: 55 mass %) GA:galvannealing layer (Fe content: 10 mass %) EG: Zn electroplating layerZN: Zn-Ni-alloy electroplating layer (Ni content: 12 mass %)

It is clarified that all the zinc-based coated steel sheets (examples ofthe present disclosure), which were manufactured by performing asurface-cleaning treatment in the manufacturing method according to thepresent disclosure, were excellent in terms of surface appearance. Inparticular, it is clarified that the surface appearance was absolutelyexcellent in the case where a Zn—Ni-alloy-electroplated steel sheet wasused. On the other hand, the zinc-based coated steel sheet (comparativeexample), which was subjected to degreasing with a strong alkali havinga pH of 12.5 or more, and the zinc-based coated steel sheet (comparativeexample), which was not subjected to a surface-cleaning treatment, werepoor in terms of surface appearance.

1. A method for manufacturing a hot-pressed member having an excellentsurface appearance, the method comprising: heating a zinc-based coatedsteel sheet to a temperature range from an Ac3 transformationtemperature to 1000° C., performing hot pressing work, and performingcooling, wherein a surface-cleaning treatment is performed on thezinc-based coated steel sheet before the heating step is performed.
 2. Amethod for manufacturing a hot-pressed member having an excellentsurface appearance, the method comprising: performing cold pressing workon a zinc-based coated steel sheet, heating the zinc-based coated steelsheet to a temperature range form an Ac3 transformation temperature to1000° C., and cooling the heated steel sheet, wherein a surface-cleaningtreatment is performed on the zinc-based coated steel sheet before theheating step is performed.
 3. The method for manufacturing a hot-pressedmember according to claim 1, wherein; the zinc-based coated steel sheetis a Zn—Ni-alloy-coated steel sheet having a coating layer on one orboth sides of the Zn—Ni-alloy-coated steel sheet, and the coating layerhas a chemical composition containing 10 mass % to 25 mass % of Ni withthe balance being Zn and inevitable impurities, and a coating weight ofthe coating layer per side of the zinc-based coated steel sheet is 10g/m² to 90 g/m².
 4. The method for manufacturing a hot-pressed memberaccording to claim 2, wherein: the zinc-based coated steel sheet is aZn—Ni-alloy-coated steel sheet having a coating layer on one or bothsides of the Zn—Ni-alloy-coated steel sheet, and the coating layer has achemical composition containing 10 mass % to 25 mass % of Ni with thebalance being Zn and inevitable impurities, and a coating weight of thecoating layer per side of the zinc-based coated steel sheet is 10 g/m²to 90 g/m².